How do I sign in to my iMessage?

If you get an error when trying to activate iMessage or FaceTime - Apple Support

What happens when you try? Error message(s)?

Messages Troubleshooting.   

Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.